LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Josh Don <joshdon@google.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Paul Turner <pjt@google.com>, Oleg Rombakh <olegrom@google.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Steve Sistare <steven.sistare@oracle.com>,
	Tejun Heo <tj@kernel.org>, Rik van Riel <riel@surriel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] sched: adjust SCHED_IDLE interactions
Date: Thu, 12 Aug 2021 14:09:15 -0700	[thread overview]
Message-ID: <CABk29Ns8P9AGy7Tpo6duOeEh=ZFWM1jO8FnvhZhktfcA0GWOpw@mail.gmail.com> (raw)
In-Reply-To: <CAKfTPtCTjhpkYz_eVr0LxcJavh__KHn2zOudD=QB5gKYZK8DtQ@mail.gmail.com>

> > @@ -697,8 +699,18 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >                 slice = __calc_delta(slice, se->load.weight, load);
> >         }
> >
> > -       if (sched_feat(BASE_SLICE))
> > -               slice = max(slice, (u64)w);
> > +       if (sched_feat(BASE_SLICE)) {
> > +               /*
> > +                * SCHED_IDLE entities are not subject to min_granularity if
> > +                * they are competing with non SCHED_IDLE entities. As a result,
> > +                * non SCHED_IDLE entities will have reduced latency to get back
> > +                * on cpu, at the cost of increased context switch frequency of
> > +                * SCHED_IDLE entities.
> > +                */
>
> Ensuring that the entity will have a minimum runtime has been added to
> ensure that we let enough time to move forward.
> If you exclude sched_idle entities from this min runtime, the
> sched_slice of an idle_entity will be really small.
> I don't have details of your example above but I can imagine that it's
> a 16 cpus system which means a sysctl_sched_min_granularity=3.75ms
> which explains the 4ms running time of an idle entity
> For a 16 cpus system, the sched_slice of an idle_entity in your
> example in the cover letter is: 6*(1+log2(16))*3/1027=87us. Of course
> this become even worse with more threads and cgroups or thread with
> ncie prio -19
>
> This value is then used to set the next hrtimer event in SCHED_HRTICK
> and 87us is too small to make any progress
>
> The 1ms of your test comes from the tick which could be a good
> candidate for a min value or the
> normalized_sysctl_sched_min_granularity which has the advantage of not
> increasing with number of CPU

Fair point, this shouldn't completely ignore min granularity. Something like

unsigned int sysctl_sched_idle_min_granularity = NSEC_PER_MSEC;

(and still only using this value instead of the default
min_granularity when the SCHED_IDLE entity is competing with normal
entities)

> > @@ -4216,7 +4228,15 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> >                 if (sched_feat(GENTLE_FAIR_SLEEPERS))
> >                         thresh >>= 1;
> >
> > -               vruntime -= thresh;
> > +               /*
> > +                * Don't give sleep credit to a SCHED_IDLE entity if we're
> > +                * placing it onto a cfs_rq with non SCHED_IDLE entities.
> > +                */
> > +               if (!se_is_idle(se) ||
> > +                   cfs_rq->h_nr_running == cfs_rq->idle_h_nr_running)
>
> Can't this condition above create unfairness between idle entities ?
> idle thread 1 wake up while normal thread is running
> normal thread thread sleeps immediately after
> idle thread 2 wakes up just after and gets some credits compared to the 1st one.

Yes, this sacrifices some idle<->idle fairness when there is a normal
thread that comes and goes. One alternative is to simply further
reduce thresh for idle entities. That will interfere with idle<->idle
fairness when there are no normal threads, which is why I opted for
the former. On second thought though, the former fairness issue seems
more problematic. Thoughts on applying a smaller sleep credit
threshold universally to idle entities?

  reply	other threads:[~2021-08-12 21:09 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30  2:00 [PATCH v2 0/2] SCHED_IDLE extensions Josh Don
2021-07-30  2:00 ` [PATCH v2 1/2] sched: cgroup SCHED_IDLE support Josh Don
2021-08-03  2:14   ` jun qian
2021-08-03 20:37     ` Josh Don
2021-08-05 10:18   ` Peter Zijlstra
2021-08-05 17:13     ` Tejun Heo
2021-08-05 23:54       ` Josh Don
2021-08-11 13:48   ` Vincent Guittot
2021-08-23  9:26   ` [tip: sched/core] sched: Cgroup " tip-bot2 for Josh Don
2021-07-30  2:00 ` [PATCH 2/2] sched: adjust SCHED_IDLE interactions Josh Don
2021-08-11 13:31   ` Vincent Guittot
2021-08-12 21:09     ` Josh Don [this message]
2021-08-13 12:43       ` Vincent Guittot
2021-08-13 23:55         ` Josh Don
2021-08-16 12:31       ` Peter Zijlstra
2021-08-17 23:48         ` Josh Don
2021-08-16 12:52     ` Peter Zijlstra
2021-08-16 12:56       ` Vincent Guittot
2021-08-17 23:40       ` Josh Don
2021-08-16 12:31   ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CABk29Ns8P9AGy7Tpo6duOeEh=ZFWM1jO8FnvhZhktfcA0GWOpw@mail.gmail.com' \
    --to=joshdon@google.com \
    --cc=bristot@redhat.com \
    --cc=bsegall@google.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=juri.lelli@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mgorman@suse.de \
    --cc=mingo@redhat.com \
    --cc=olegrom@google.com \
    --cc=peterz@infradead.org \
    --cc=pjt@google.com \
    --cc=riel@surriel.com \
    --cc=rostedt@goodmis.org \
    --cc=steven.sistare@oracle.com \
    --cc=tj@kernel.org \
    --cc=vincent.guittot@linaro.org \
    --cc=viresh.kumar@linaro.org \
    --subject='Re: [PATCH 2/2] sched: adjust SCHED_IDLE interactions' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).